In view of the increasing demand from environmental regulations, banning tetraethyl lead (TEL) from aviation gasoline (Avgas) formulations is a question of time.
Thus, several companies and research groups are presently involved in projects aiming at providing fuel formulations having a performance similar to those using TEL.
Such endeavor is not an easy task since such product should be endowed with special features among which high octane rating, stability and high performance are among the most important items required for the new product.
Alternative formulations described in the literature employ oxygenated compounds (ethyl tert-butyl ether—ETBE, ethyl alcohol) and aromatic amines, chiefly meta-toluidine (m-Tol) as components of the new fuels, all of them still being developed or tested and none of them being yet commercially available.
Aviation gasoline fuels are mainly used in small passenger aircraft for leisure activities and also in small piston driven helicopters.
Because of the limited consuming market, aviation gasoline is produced in few refineries where the required stringent specifications are met without affecting too much the overall refinery budget.
The development of a specific aviation gasoline fuel started in 1930. At that time the air sector of the US Army specified a combat gasoline fuel having a minimum octane number requirement of 87. This is believed to be the first time the anti-knocking properties of an aviation gasoline fuel were defined in terms of octane number. In the beginning of Second World War fuels were similar to those of present class 100.
Aviation gasoline attained its development peak during Second World War. In 1944, the US Army issued a specification for class 115/145. Such fuel had the highest anti-knocking evaluation among all commercial aviation gasoline and was used to obtain maximum response of high performance engines.
Useful streams for this kind of fuel should have a narrow distillation range and high octane numbers. C5 and C6 isoparaffin cuts—those being used in small concentrations due to their high volatility—or alkylates are generally sought for these formulations.
Suitable additives are frequently the same as those used in jet engine fuels. However, the use of tetraethyl lead (TEL) is specific for aviation gasoline fuels. The use of this additive is specified in ASTM Method D 910 and the amounts are a function of the fuel class. Maximum allowed levels are 0.14 g/L, 1.12 g/L, and 0.56 g/L, respectively for 80/87, 100/130, and 100/130 LL classes. The only difference between the last two gasoline fuels is color and lead amount, with LL having the meaning of low lead.
According to several fuel experts, for economic reasons the consumption of gasoline fuel for light aviation (100 LL gasoline fuel) can be suppressed in the next years. Without a substitute, many aircraft will no longer be able to fly. The main reason for this concern is the quick decline in other TEL applications and a corresponding drop in the supply.
A few years ago there were nearly 40-50 TEL suppliers while nowadays only two are left, one in the United Kingdom and another one in Russia.
In spite of the research under course, at this moment there is no unleaded additive for Avgas.
In spite of the rather modest contribution of Avgas for lead pollution, there is a huge concern of environmental agencies for banning such lead source from the atmosphere.
Lead-related additives constitute a further problem.
Ethylene dibromide, another cleaning additive used with TEL in 100 LL gasoline fuel, is highly toxic and should also be banned since it is a chemical product leading to the greenhouse effect. Without this cleaning additive, lead can block the engine in a few hours.
Low consumption, soaring prices, added to the pressure from environmental agencies are parameters that tend to cause the end of leaded Avgas around 2005-2010. The same reasons make the basis for the research of a new product for an unleaded gasoline fuel.
The patent literature teaches a few low lead or unleaded Avgas formulations.
U.S. Pat. No. 6,767,372 discloses an aviation gasoline (Avgas) fuel composition possessing a high motor octane number containing reduced amounts of tetraethyl lead. The Avgas composition preferably comprises about 20 to about 80 vol % iso-octane, about 5 to about 18 vol % toluene, about 1 to about 20 vol % C4 to C5 paraffins, about 0 to about 1 mL/gallon tetraethyl lead (TEL) and the balance light alkylate. The Avgas composition may be economically produced utilizing spare methyl tertiary butyl ether plant capacity to produce iso-octane as one component of the composition.
U.S. Pat. No. 6,451,075 relates to a particularly useful low lead aviation gasoline fuel blend that complies with all requirements of ASTM D 910 and includes 67.0 volume % isooctane, 18.0 volume % xylene, 12.0 volume % isopentane, 3.0 volume % isobutane and 0.47 mL/gal of tetraethyl lead. Another useful low lead aviation gasoline fuel blend which complies with all requirements of ASTM D 910 except for oxygenate content includes 60.0 volume % isooctane, 15.0 volume % xylene, 14.0 volume % methyl t-butyl ether, 8.0 volume % isopentane, 3.0 volume % isobutane and 0.2 mL/gal tetraethyl lead. The isooctane used in either case is a purified isooctane prepared either by fractionating a crude DIB (diisobutylene) material and hydrogenating the fractionated material or hydrogenating the crude DIB material and fractionating the hydrogenated material.
U.S. Pat. No. 6,258,134 and U.S. Pat. No. 5,851,241 relate to aviation fuel compositions that contain a substantially positive or synergistic combination of an alkyl tertiary butyl ether, an aromatic amine and, optionally, a manganese component. The base fuel containing the additive combination may be a wide boiling range alkylate. MON is at least 91, and is raised to at least 94 by the addition of the cited components.
U.S. Pat. No. 6,238,446 teaches unleaded aviation gasoline having heats of combustion and octane rating deemed necessary for use under actual service conditions that are formed from blends of specific proportions of aviation alkylate, ether blending agent, a cyclopentadienyl manganese tricarbonyl and optionally other appropriate hydrocarbons falling in the gasoline boiling range.
U.S. Pat. No. 5,470,358 relates to an unleaded aviation gasoline fuel, containing additives such as aromatic amines substituted with C1-C10 alkyl or halogen, where the alkyl substituent cannot occupy the 2- or 6-position in the phenyl ring. According to this document, Avgas, due to stringent requirements in terms of octane rating and stability, is a blend of isopentane, alkylate, toluene and TEL. A base fuel useful as Avgas without octane booster such as TEL has a MON rate of 90 to 93. MON, and not RON, is the accepted parameter for the Avgas octane and is measured using the ASTM Method 2700-92. The additive is used in amounts from 6 to 10 wt %, if necessary with the aid of a co-solvent.
However, in spite of the present developments, the technique still needs an alkylate-based unleaded Avgas formulation, which contains triptane (2,2,3-trimethyl butane) associated to toluidine and super-alkylate in suitable proportions to lead to an Avgas having suitable properties, such Avgas formulation being described and claimed in the present application.